Computers and Control Systems: Description and Operation
DESCRIPTIONThe Powertrain Control Module (PCM), is located under the passenger side of the dash. It is the control center of the fuel injection system and constantly looks at the information from various sensors and controls the systems that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL) Service Engine Soon, and store a Diagnostic Trouble Code (DTC) which identifies the problem areas to aid the technician in making repairs.
MALFUNCTION INDICATOR LAMP
The MIL (Service Engine Soon/Check Engine) lamp is on the instrument panel and has the following functions:
^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
^ Some applications will flash diagnostic trouble codes stored by the PCM which help the technician diagnose system problems.
^ Some applications will flash "Open Loop" or "Closed Loop" operation.
Refer to Computers and Controls / Testing and Inspection / Vehicle On-Board Diagnostics / Without Scan Tool for information on identifying exactly what information the MIL can provide. Testing and Inspection
As a bulb and system check, the MIL (Service Engine Soon/Check Engine) will come "ON" with the key "ON" and the engine not running. When the engine is started, the lamp will turn "OFF." If the lamp remains "ON," the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases but a diagnostic trouble code will remain stored.
When the light remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic System Check must be performed. The procedures for these checks is located at Computers and Controls / System Diagnosis / Flow of Diagnosis. Testing and Inspection
These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.
DIAGNOSTIC INFORMATION
There is a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language for communicating the source of a malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the PCM, a diagnostic trouble code is set and the MIL is illuminated on some applications.
LEARNING ABILITY
The system has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected, to clear diagnostic trouble codes or for other repair the "learning" process resets and begins again. A change may be noted in the vehicle's performance. To "teach" the vehicle, ensure that the engine is at operating temperature. The vehicle should be driven at part throttle, with moderate acceleration and idle conditions until normal performance returns.
SHORT TERM FUEL TRIM
Short term fuel trim is a PCM volatile memory register that will contain a number between 0 and 255. The neutral value for the short term fuel trim is 128; any deviation from this value indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 128. The short term fuel trim changes the pulse width by varying the "Closed Loop" factor of the base pulse width equation. As the PCM monitors the Heated Oxygen Sensor (HO2S) input, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term fuel trim trends. The correction of long term fuel trim trends is the function of the long term fuel trim.
LONG TERM FUEL TRIM
The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what "long term fuel trim" factor to use in the base pulse width equation.
While in any given block, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 128, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed it should force the short term fuel trim back toward 128.
If the mixture is still not correct (as judged by the HO2S), the short term fuel trim will continue to have a large deviation from the ideal 128. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced.
Both the short term fuel trim and long term fuel trim are limited by calibrated values. If the air/fuel is off enough so that long term fuel trim reaches the limit of its control and still cannot correct the condition, the short term fuel trim would also go to its limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at their extreme values, a DTC PO131 or PO132 will likely result. Under the conditions of power enrichment, the PCM sets the short term fuel trim to 128 and freezes it there until power enrichment is no longer in effect. This is done so the "Closed Loop" factor and the long term fuel trim will not try to correct for the commanded richness of power enrichment.